A standard lab hood is an enclosure one side of which can be opened by means of a movable door, typically an upwardly slidable glass panel. The top of the hood is connected via a branch duct to a main vent duct that is normally continuously evacuated by a vent fan. The branch duct is provided with a shutter that can be moved from a position completely blocking air flow up the branch duct to a position exposing virtually the entire flow cross section of the branch duct for maximum flow. The main duct is maintained continuously under a predetermined subatmospheric pressure.
In order to avoid running the exhaust system at a given hood it is known to mechanically couple the door of the hood to the respective shutter so that, as the door is closed the shutter is closed and vice versa. This ensures that when the door is open there will be sufficient suction to prevent any escape of gases from the hood. When, however, the door is closed there will be little venting and, therefore, little waste of heated or cooled room air and of energy running the suction source. The suction control can be gradual, controlled by a pontentiometer or the like operated as the door is raised and lowered, or two-stage, controlled by end switches operated by the door. At best such systems approximate ideal circumstances.